Tungsten-halogen lamps employing a halogen regenerative cycle are known. Such lamps operate with virtually 100% lumen maintenance over their service life because the halogen presence keeps evaporated tungsten from depositing on the bulb wall and blackening the same.
Originally, the light emitting capsules, which are quite small compared with a conventional incandescent envelope, were made from fused silica or quartz. Subsequent generations have employed various hard glasses, such as borosilicate and aluminosilicate. For the most part, tungsten-halogen lamps have been used in special purpose applications because of their relatively high cost. Generally, the lamp capsule exists, and is usable, by itself; occasionally, however, they have been contained in an outer envelope. Halogen headlamps for automobiles are an example of the latter.
When enclosed within an outer envelope, the light emitting capsule is generally mounted upon a stem or mounting structure, the latter structure being hermetically sealed to the outer envelope. This provides a relatively unstable and fragile construction because, during shipping, the heavy capsule would cause the lead-in wires to bend allowing the capsule to move over toward the bulb wall. Under certain conditions this could cause the outer envelope to break.
Furthermore, it generally is necessary to employ the same type of glass for the capsule, the stem and the outer envelope in order to match thermal expansion coefficients and sealing capabilities. This leads also to added expense since quartz or fused silica is much more costly than hard glass, and the latter is more expensive than soft glasses such as the common soda-lime glasses.